To convert from Python arrays to PostgreSQL quickly?

问题

This is a follow-up question to: How to cast to int array in PostgreSQL?
I am thinking how to convert Python's datatype of array-array of signed integer into to int of PostgreSQL quickly:

import numpy as np; # use any data format of Python here
event = np.array([[1,2],[3,4]]);

where [] should be replaced by {} and surrounded by ' if manually. In PostgreSQL, the following is accepted as the syntax of the datatype

...
FOR EACH ROW EXECUTE PROCEDURE insaft_function('{{1,2},{3,4}}');

@JohnMee's suggestion

str(event).replace('[','{').replace(']','}').replace('\n ',',')

@ErwinBrandstetter's suggestion

Stick to signed integers because it is supported by SQL standard. Map to int, so just in PostgreSQL side:

TG_ARGV::int[]

I want to stick to this Erwin's suggestion.

Test run of simpler version of @ErwinBrandstetter's answer

I have to simplify his answer to keep it enough focused here by removing the table-name from function so just keeping the trigger for one initial table measurements:

CREATE OR REPLACE FUNCTION f_create_my_trigger(_arg0 text)
  RETURNS void AS
$func$
BEGIN
EXECUTE format($$
    DROP TRIGGER IF EXISTS insaft_ids ON measurements;
    CREATE TRIGGER insaft_ids
    AFTER INSERT ON measurements
    FOR EACH ROW EXECUTE PROCEDURE insaft_function(%1$L)$$
    , _arg0
);

END
$func$ LANGUAGE plpgsql;

And I run:

sudo -u postgres psql detector -c "SELECT f_create_my_trigger('[[1,2],[3,4]]');"

But get empty output:

 f_create_my_trigger 
---------------------

(1 row)

How can you map to int for PostgreSQL 9.4 in Python?

回答1:

Summing up the situation

You want to create triggers (repeatedly?) using the same trigger function like outlined in my previous answer on dba.SE. You need to pass values to the trigger function to create multiple rows with multiple column values, hence the two-dimensional array. (But we can really work with any clearly defined string!)

The only way to pass values to a PL/pgSQL trigger function are text parameters, which are accessible inside the function as 0-based array of text TG_ARGV[]. You could pass a variable number of parameters, but we discussed a single string literal representing your 2-dimenstional array earlier.

The values you want to pass come from a 2-dimenstional Python array with signed integer numbers, so they will fit into the Postgres integer type. You would have to use the Postgres type bigint for unsigned integer numbers to be sure, like I commented.

The text representation in Python looks like this:

[[1,2],[3,4]]

The syntax for a Postgres array literal looks like this:

{{1,2},{3,4}}

And it seems like you want to automate the process.

Full automation

You can concatenate the string for the CREATE TRIGGER statement in your client or you can persist the logic in a server-side function and just pass parameters.

Demonstrating an example function where we pass the table name and the string that's used as parameter to the trigger function. The trigger function insaft_function() is defined in your previous question on dba.SE.

CREATE OR REPLACE FUNCTION f_create_my_trigger(_tbl regclass, _arg0 text)
  RETURNS void AS
$func$
BEGIN

EXECUTE format($$
   DROP TRIGGER IF EXISTS insaft_%1$s_ids ON %1$s;
   CREATE TRIGGER insaft_%1$s_ids
   AFTER INSERT ON %1$s
   FOR EACH ROW EXECUTE PROCEDURE insaft_function(%2$L)$$
   , _tbl, translate(_arg0, '[]', '{}')
   );

END 
$func$ LANGUAGE plpgsql;

Call:

SELECT f_create_my_trigger('measurements', '[[1,2],[3,4]]');

Or:

SELECT f_create_my_trigger('some_other_table', '{{5,6},{7,8}}');

SQL Fiddle demo.

Now you can pass either [[1,2],[3,4]] (with square brackets) or {{1,2},{3,4}} (with curly braces). Both work the same. translate(_arg0, '[]', '{}' transforms the first into the second form.

This function automatically drops a trigger of the same name if it exists, before creating the new one. You may want to drop or keep this line:

DROP TRIGGER IF EXISTS insaft_%1$s_ids ON %1$s;

This runs with the privileges of the calling DB role. You could make it run with superuser (or any other) privileges if need be.

There are many other ways to achieve this. It all depends on exact requirements.

Explaining format()

format() and the data type regclass help to safely concatenate the DDL command while making SQL injection impossible. Details:

Table name as a PostgreSQL function parameter

The first argument is the "format string", followed by arguments to be imbedded in the string. I use dollar-quoting, which is not strictly necessary for the example, but generally a good idea for concatenating long strings that might contain literal quotes: $$DROP TRIGGER ... $$

format() is modeled along the C function sprintf. %1$s is a format specifier of the format() function. It means that the first (1$) argument after the format string is inserted as unquoted string (%s), hence: %1$s. The first argument to format is _tbl in the example - the regclass parameter is rendered as legal identifier automatically, double-quoted if necessary, so format() does not have to do more. Hence just %s, not %I (identifier). Read the linked answer above for details.
The other format specifier in use is %2$L: Second argument as quoted string literal.

If you are new to format(), play with these simple examples to understand:

SELECT format('input -->|%s|<-- here', '[1,2]')
     , format('input -->|%s|<-- here', translate('[1,2]', '[]', '{}'))
     , format('input -->|%L|<-- here', translate('[1,2]', '[]', '{}'))
     , format('input -->|%I|<-- here', translate('[1,2]', '[]', '{}'));

And read the manual.

来源：https://stackoverflow.com/questions/31533496/to-convert-from-python-arrays-to-postgresql-quickly

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python

arrays

postgresql

triggers

plpgsql